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chromium/third_party/blink/web_tests/wpt_internal/html/semantics/forms/the-select-element/stylable-select/select-open-invalidation-ref.html 

<!DOCTYPE html>
<link rel=stylesheet href="resources/stylable-select-styles.css">

<style>
button {
  color: green;
}
</style>

<div id=container class=stylable-select-container>
  <button>button</button>
  <div id=popover popover=auto anchor=container class=stylable-select-datalist>
    <div tabindex=0 class="stylable-select-option selected">one</div>
    <div class=stylable-select-option>two</div>
  </div>
</div>

<script>
document.getElementById('popover').showPopover();
</script>